A water-absorbent resin has been widely used for hygiene products such as disposable diapers or sanitary products, products for daily use such as sheets for pets, industrial materials such as water-absorbent sheets for food, water-blocking materials for cables, or dew-catchers, water retention agents for greening, agriculture, and gardening, soil conditioners, and the like. This water-absorbent resin is particularly used for hygiene products among the above-described uses.
Typically, such a water-absorbent resin is a lightly-crosslinked polymer. For example, a starch-based water-absorbent resin such as a starch-acrylonitrile graft copolymer hydrolysate (refer to Patent Literature 1) or a starch-acrylic acid graft copolymer neutralized product (refer to Patent Literature 2), a vinyl acetate-acrylic acid ester copolymer saponified product (refer to Patent Literature 3), and a partially neutralized product of polyacrylic acid (refer to Patent Literatures 4, 5, and 6) are known.
The water-absorbent resin is produced as a particulate powder by polymerizing a water-soluble ethylenically unsaturated monomer in a polymerization reactor to obtain a water-absorbent resin composition and drying the obtained water-absorbent resin composition in a dryer.
During the production of the water-absorbent resin, the water-absorbent resin as a polymer of the water-soluble ethylenically unsaturated monomer may adhere on, for example, an inner wall surface of the polymerization reactor or the dryer, and this adhered material may function as a core such that the water-absorbent resin powder grows and aggregates to produce a powder aggregate. The properties of the powder aggregate as the water-absorbent resin are likely to be poor. Therefore, when the water-absorbent resin is produced as a particulate powder, it is necessary to remove this powder aggregate.
As a method of removing the powder aggregate during the production of the water-absorbent resin, a method of drying the water-absorbent resin powder and then classifying the dry powder using a classifier is generally known (refer to Patent Literature 7).